The law of treaty interpretation aspires to unity. All treaties are formally subject to the same rules of interpretation, codified in the Vienna Convention. Yet time and again we hear that some kinds of treaties are entitled to special treatment. Most commonly the idea is that certain exceptional conventions are capable of evolving, with or without the continued consent of the parties — as with certain human rights conventions. Other times the claim is that certain kinds of agreements resist techniques of interpretation that establish treaty change over time. To date, explanations for such differential treatment remain unsatisfying. This Chapter seeks to better account for the practice of affording some treaties special status. I argue that differential treatment cannot be justified by appeal to a treaty’s object and purpose alone, but must be understood in light of the nature of the obligations that the parties established to achieve their goals.